Electromagnetic clutches are commonly used on automotive vehicles to drive auxiliary equipment such as air conditioning compressors, air compressors and superchargers. These electromagnetic clutches have a rotor and an armature each of which includes a plurality of arc-shaped slots. The arc-shaped slots are radially spaced from the axis of rotation of the armature and the rotor and extend generally in arcuate paths about the axis of rotation. The arc-shaped slots on the rotor cooperate with the arc-shaped slots on the armature to form magnetic path discontinuities. An electromagnetic coil is mounted adjacent to the rotor. When the electromagnetic coil is energized, the armature and the rotor are attracted to each other, moved into contact and engage the clutch. The rotor is usually driven by a belt that is trained around a belt engaging surface on the rotor and a drive pulley driven by an engine or a motor. The armature is rotationally coupled to an auxiliary device and transmits torque to the auxiliary device from the belt when the electromagnetic clutch is engaged. An abnormally high torque load on the auxiliary device will cause slippage between the rotor and the armature. Slippage between the rotor and the armature generates heat. The heat will damage the belt, a bearing or other parts if slip between the rotor and the armature continues for more than a short period of tine.
Automotive vehicles in use today are frequently equipped with a single belt that transmits power to the rotor of an electromagnetic clutch to drive auxiliary equipment, and to critical components such as water pumps, air compressors, alternators and power steering pumps. With these vehicles a damaged belt or bearing due to a slipping electromagnetic clutch will result in the loss of power to a critical component and the vehicle may become disabled.
Systems to detect slippage between the rotor and the armature of an electromagnetic clutch have been proposed. Upon detecting slippage, the electromagnetic clutch can be disengaged to eliminate clutch damage and to prevent a vehicle from becoming disabled. Systems for detecting slip between the armature and a rotor have sensed the flow of current in an electromagnetic coil in the clutch. The "Electromagnetic Frictional Clutch Device" disclosed in Laid-Open Japanese Patent No. 205030/1984, for example, senses a current induced in the clutch electromagnetic coil by a variation in inductance or magnetic flux in a magnetic circuit to detect slip. The electromagnetic clutch has a yoke in which an electromagnetic coil is accommodated, a rotor with an annular groove that receives the yoke, a frictional surface on the rotor with a plurality of notches or grooves that extend radially at circumferentially spaced intervals, and an armature that can be magnetically attracted to the rotor friction surface which has the same number of slits or grooves as the number of notches or grooves in the armature friction surface. The current flowing in the electromagnetic coil varies when the clutch is engaged depending on the state of overlap of the notches and the rotor and the slits in the armature. Slip between the rotor and the armature is detected by detecting changes in the current flowing through the electromagnetic coil. When slip between the armature and the rotor is detected, the clutch is disengaged.
U.S. Pat. No. 4,949,828 to Olsen discloses another electromagnetic clutch with a rotor and an armature with slots that create magnetic discontinuities so that the rotor and armature cooperate to form a magnetic circuit when the coil is energized and pulls the armature and the rotor into engagement. The slots in the rotor and the armature have a special shape so that slippage between the armature and the rotor produces an alternating current in the direct current flowing in the electromagnetic coil. The presence of the alternating current component can be sensed through a rectifier or frequency filter. The alternating current component is compared with a predetermined threshold in the supply of current and the electromagnetic coil is disconnected from a current supply if the threshold is exceeded.
Detecting slip based on peak magnetic noise is difficult in any system powered by an internal combustion engine. Slip or electrical noise occurring in the low speed rotation range of an engine and rotation noise occurring in the high speed rotation range of an engine are difficult to discriminate between. The operation of other electrical parts can be amplified and their electrical disturbances erroneously recognized as slip. Failure to discriminate between electrical disturbances from the clutch armature and rotor and sources other than the armature and rotor can impair slip detection and render a slip detector unreliable.